Down hole periodic seismic generator

ABSTRACT

A down hole periodic seismic generator system for transmitting variable frequency, predominantly shear-wave vibration into earth strata surrounding a borehole. The system comprises a unitary housing operably connected to a well head by support and electrical cabling and contains clamping apparatus for selectively clamping the housing to the walls of the borehole. The system further comprises a variable speed pneumatic oscillator and a self-contained pneumatic reservoir for producing a frequency-swept seismic output over a discrete frequency range.

FIELD OF INVENTION

The present invention relates to the generation of seismic signals foruse in determining the structural characteristics of earth strata andmore particularly, but not by way of limitation, to a down hole periodicseismic generator for the transmission of variable swept-frequencypredominantly shear-wave vibrations into earth strata formationssurrounding a bore hole.

HISTORY OF THE PRIOR ART

Methods of conducting seismic exploration include the production andtransmission of seismic waves through the earth's surface and thepositioning of geophone receivers at strategic locations in the area ofinterest for receiving the reflected signals. These reflected signalsare then correlated with the source of the seismic shock waves in aneffort to determine the characteristics of the earth's strata in thearea of interest.

Explosive charges have been widely used as a source of these seismicwaves but have several disadvantages, one of which is the unpredictablecharacteristics of the explosive sources.

With the development of sophisticated receiving equipment, it has becomefeasible to induce low frequency vibration signals into the earthwhereupon, the reflected signal is electronically correlated to thesource signal. A primary method of inducing earth vibration has been toproduce shear waves by vibrating a surface base plate which is incontact with the earth. However, in order to obtain the neededvibrational force, extremely large vehicles have been necessary in orderto hold the base plate against the earth.

Another problem associated with this method is that of attenuation ofthe signal as it passes through the softer surface layers of the earth,hence, greatly limiting the effective depth of exploration. In fact, ithas been determined that periodic seismic sources generated at thesurface lose as much as two-thirds of the input energy in surface orRayleigh waves while a significant portion of the remaining signal madeup of pressure (p) waves and shear (s) waves are attenuated in theporous surface layers of the earth. These porous surface layers oftenextend to depths as much as 3000 feet.

Once below surface layers, shear waves propagate very well to producevaluable seismic information. It has been found that shear wavesreflected at significant depths are extremely useful in detectingunderground gases and liquids such as molten magma, geothermal fluids,fossil fuels and the like.

In order to overcome this inherent disadvantage, various attempts havebeen made to position a seismic source inside a bore hole at depthsbelow the surface porous layers. Typical of these devices are found inthe teachings of the patent to Malmberg, U.S. Pat. No. 3,221,833 issuedDec. 7, 1965, for a "Geophysical Bore Hole Apparatus" and the patent toFair et. al., U.S. Pat. No. 3,718,205 issued Feb. 27, 1973, for a "Borehole Seismic Transducer". However, such systems have enjoyed limitedsuccess through ineffective coupling with the bore hole wall and theutilization of fluid drive sources located at the wellhead requiringelaborate down hole plumbing.

The Patent to Brooding et. al., U.S. Pat. No. 3,909,776 issued Sept. 3,1977, entitled "Fluid Oscillator seismic Source" recognized the value ofinducing a variable frequency seismic source down hole, but again, washampered by the fluid drive source being located at the wellhead andhaving to maintain fluid pressure throughout an elongated pipe orannulus.

A further disadvantage occurring with the use of a variable frequencyseismic oscillator is that typically as the frequency of the oscillatorpiston or "hammer" increases, the power of the seismic pulse decreasesthereby resulting in an undesirable decrease in amplitude of theresulting seismic wave over the frequency sweep.

When wellhead equipment is used to provide the fluid drive source, thelow signal to noise often requires at the surface equipment consistingof pumps and the like be shut down during the time that the reflectedsignals are being received which further complicates and hampers the useof existing down hole seismic generators.

It has been suggested in the art that the problem of signal attenuationcould be relieved by drilling several bore holes at strategic locationsin the area of interest. A periodic seismic source is then lowered intoone of the bore holes while geophone receivers are lowered into theothers. While this system appears to have merit, the problem withexisting down hole seismic sources as here before pointed out are stillprevalent.

SUMMARY OF THE INVENTION

The present invention provides a down hole seismic generator system fortransmitting variable frequency seismic shear waves into earth strataformations surrounding a bore hole and which is designed to overcome theabove disadvantages.

This system comprises an elongated unitary housing which is clampable tothe bore hole wall and operably connected to an electrical controldevice at the wellhead by electrical cables and support cables. Thesystem is compatible with commercially available standard logging cablecontaining both electrical wires and support lines in a single cable.

The housing contains a seismic generator which generally comprises aself-contained pneumatic reservoir which is charged with gas such as airbefore lowering the generator into the bore hole. The reservoir isoperably connected to a reciprocating piston or hammer through avariable speed electrically driven valve which is controlled from thewellhead.

A clamping apparatus is provided at the lower portion of the housinghaving oppositely disposed transversely extendable shoe members whichmay be extended outwardly into rigid engagement with the bore hole wallfor conducting seismic vibration from the reciprocating hammer into thesurrounding earth strata.

The pneumatic reservoir comprises an elongated cylinder having a two-waydrive piston disposed thereon. This drive piston is mechanicallyconnected to an electric motor capable of driving the piston in eitherdirection. Each end of the cylinder is provided with air passagewayswhich are in communication with the reciprocating hammer through ahammer control valve. The hammer control valve is in turn operablyconnected to the aforementioned variable speed motor.

The reciprocating hammer is freely carried in an elongatedlongitudinally disposed chamber, each end of which is connected by portsand passageways to the hammer control valve. The hammer is made tovibrate by pressurized air forced from the reservoir and being appliedalternately to each side of the hammer chamber. When pressurized air isapplied to one end of the hammer chamber, the opposite end is ported tothe low pressure side of the pneumatic drive piston. Therefore, when thepneumatic drive piston has traveled the full length of the cylinder, itsdirection may be reversed to provide that same gas under pressure duringa reverse stroke.

As herein before stated, it has been found that as the frequency of thehammer increases, there is a power drop thereby reducing the outputamplitude of the seismic waves generated by the hammer. This problem issignificantly reduced or eliminated by the design of the reciprocatinghammer and hammer chamber. The configuration of the hammer and sizing ofthe air passageways associated therewith are designed such that aportion of the air remains in the hammer during exhaust to provide acushion or spring effect. This feature is combined with selection of ahammer mass to produce a resonant frequency just beyond the maximumoperating frequency of the generated seismic wave.

Therefore, as the oscillating frequency of the hammer approaches theupper end of its frequency range, the hammer approaches resonance,thereby resulting in an increased ratio of output energy to input energyso that although the input energy is decreasing, the output energy isbeing maintained somewhat constant.

The clamping apparatus generally comprises a pair of outwardlyextendable shoe members having serrated outer surfaces for engagingeither the wall of the bore hole or a casing member carried therein. Theshoe members are connected to linkage arms which are in turn carried byan elongated screw rod. The rod is coupled to the shaft of a reversibleservo motor for operating in either direction and locking in any desiredposition.

Hence, the present invention provides a system which is mechanicallyself-contained needing only electrical and support cables extending tothe wellhead.

OBJECTIVES OF THE INVENTION

An objective of the invention is to provide a down hole periodic seismicgenerator system for transmitting shear wave vibrations over discretelow frequency ranges into earth strata formation surrounding a borehole. Another object of the invention is to provide an oscillatinghammer designed whereby the resonance frequency of the oscillator hammeris just above the operating frequency range to increase the efficiencyof the hammer at the higher frequency levels thereby providing arelatively flat output response.

Another objective of the invention is to provide a self-containedpneumatic reservoir for driving the oscillating hammer.

Another object of the invention is to provide a self-contained pneumaticreservoir which employs a two-way drive piston for providing airpressure to the oscillating hammer when driving the piston in eitherdirection thereby providing a down hole recyclable air supply.

Another object of the invention is to provide a down hole periodicseismic generator having a mechanical clamping apparatus which is simplein construction and efficient in operation.

DESCRIPTION OF THE DRAWINGS

Other and further advantageous features will hereinafter more fullyappear in connection with a detailed description of the drawings inwhich:

FIG. 1 is an elevational section view of a bore hole containing aseismic generator system embodying the present invention.

FIG. 2 is a schematic block diagram of the seismic generator system ofFIG. 1.

FIGS. 3A-3E are sectional segmented views of the down hole seismicgenerator wherein; FIG. 3A depicts the cable support end of thegenerator, FIGS. 3B and 3C show the self contained pneumatic reservoirsegment, FIG. 3D depicts the oscillating hammer portion, FIG. 3E depictsthe clamping apparatus with the shoe members, partially extended.

FIGS. 4 and 5 are end sectional views of the reservoir section takenalong the broken lines 4--4 and 5--5, respectively, of FIGS. 3C;

FIGS. 6A and 6B schematically depict an embodiment of a directionalcontrol valve contained in the reservoir section;

FIGS. 7-11 are end sectional views of the oscillating hammer sectiontaken along the broken lines 7--7, 8--8, 9--9, 10--10, and 11--11,respectively, of FIG. 3D;

FIGS. 12 and 13 schematically depict an embodiment of the hammer controlvalve of the oscillating hammer section;

FIG. 14 is a sectional view of the clamping apparatus of FIG. 3E withthe shoe members fully retracted;

FIG. 15 is an end sectional view of the apparatus of FIG. 14 taken alongthe broken lines 15--15;

FIG. 16 is a view of the clamping apparatus in FIG. 14 rotated 90degrees.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in detail reference character 10 generallydepicts a down hole periodic seismic generator suspended in a bore hole12 by means of a cable assembly 14 which is operably connected to awellhead control device 16. FIG. 1 generally depicts a clamp couplingmeans 18 which serves to rigidly secure the seismic generator 10 inengagement with the bore hole wall 12.

FIG. 2 schematically depicts a seismic generator system 10 which iselectrically controlled from the wellhead 16 for providing periodicseismic generated waves for receipt by one or more geophone receiversgenerally indicated by reference character 20 which may be located atstrategic positions at the surface or may be disposed in adjacent boreholes.

The down hole system generally comprises a self-contained pneumaticreservoir 22 having a piston drive motor 24 for providing gas or airunder pressure to a directional control valve 26. The directionalcontrol valve in turn supplies the pressurized gas to a longitudinallydisposed pneumatic oscillator 28, the frequency of oscillating beingcontrolled by a variable speed electric motor 30. The air after beingpassed through the oscillator 28 may be returned through the directionalcontrol valve to the low pressure side of the pneumatic reservoir forreuse in a manner that will be hereinafter set forth. The clamp coupler18 is an electro-mechanical apparatus for selectively clamping the downhole system 10 into rigid engagement with the bore hole wall whereby theenergy may be transmitted from the oscillator 28 to the earth's stratasurrounding the bore hole 12. The clamp is operated by an electricalservo-motor 32.

Referring now to FIGS. 3A through 3E, the down hole generator 10comprises an elongated cylindrical housing 34 which is physicallysupported by the cable assembly 14 through a cable coupling apparatusgenerally indicated by reference character 36. The cable assembly 14comprises both support cabling and electrical cabling, the electricalcabling comprising both electrical transmission lines for the operationfor the down hole electrical motors in addition to electrical signalcontrol lines. The support cabling is attached to the housing 34 bymeans of the coupling apparatus 36 as is well known in the art while theelectrical cables are passed through a connector indicated by referencecharacter 38 to facilitate detaching the down hole system from the cableassembly.

A first housing segment 40 contains the reversible electric motor 24which has an output rotary shaft 42. The output shaft 42 is operablyconnected to one end of an elongated threaded rod 44 by way ofmechanical coupling 46 whereby the rod 44 rotates with the motor outputshaft 42. The rod 44 is journaled in a second housing segment 48, oneend of the rod 44 extending through a partition plate 50.

A third housing segment 52 contains the pneumatic reservoir 22. A sealedpneumatic piston 54 is reciprocally disposed in the pneumatic reservoir22 and is carried by an elongated hollow piston rod 56. The upper orouter end of the piston rod 56 is provided with an internally threadedsleeve member 58 which is threadily disposed about the elongated rod 44.The sleeve member 58 and associated piston rod 56 are slidingly disposedin a gland nut 60 which in turn is carried by a block portion 62 whichseparates the housing segments 48 and 52. The sleeve member 58 isprevented from rotating with the threaded rod 44 by means of anelongated track 64 and associated finger member 66. The track 64 iscarried within the housing segment 48 while the finger member 66 isattached to the sleeve member 58. Hence, rotation of the threaded rodmember 44 causes the sleeve member 58 and associated piston rod 56 toslide longitudinally through the gland nut 60 thereby imparting areciprocal movement to the piston member 54.

A first pneumatic pressure passageway 68 is provided in the housingsegment 52, an upper end thereof being ported into the upper end of thereservoir 22 at reference character 70. The opposite end of thepassageway 68 is connected into a one-way or check valve 72 carried bythe directional control valve 26 which is disposed at the opposite orlower end of the pneumatic reservoir 22. A second return air passageway74 is provided in the housing segment 52 having one end ported to theupper end of the pneumatic reservoir 22 at reference character 76. Theopposite end of the passageway 74 is connected to a second one-way orcheck valve 78 which is also carried by the directional control valve26.

The lower end of the pneumatic passageway 22 is in communication withthe directional control valve 26 by way of a pressure passageway 80 anda return passageway 82. The pressure passageway 80 and connected to aone-way or check valve 84 while the return passageway 82 is connected toa check valve 86 both contained in the directional control valve 26. Thelower end of the directional control valve 26 is connected to a pressurepassageway 88 and a single return passageway 90.

Therefore, if the reversible electrical motor 24 is made to rotate theoperator rod 44 in a first direction, the piston 54 is moved downwardlythereby causing pressurized air to flow through the passageway 80 thecheck valve 84 and into the pressure passageway 88. The check valve 72prevents air from passing through the passageway 68 to the upper end ofthe pneumatic chamber 22. At the same time, return air from thepassageway 90 may flow through the check valve 78 in the directionalcontrol valve 26 and through the passageway 74 into the upper end of thepneumatic chamber 22.

Likewise, when the motor 24 is rotated in the opposite direction,pressurized air from the upper end of the piston passes through thepassageway 68, the check valve 72 and into the pressure passageway 88likewise, return air then flows through the check valve 78 and thereturn passageway 82.

The pressure passageway 88 and return passageway 90 are interrupted by avalve member 91 for selectively blocking the passageways. By closing thevalve 91, the position 54 may be moved in one direction in order to, ineffect, charge the pneumatic reservoir with air under pressure whereupon opening the valve 91 releases the pressurized air to the hammeroscillator section as will be hereinafter set forth.

Hence, the pneumatic reservoir and associated piston may providepressurized air with either a downward stroke of the piston 54 or on thereturn stroke. The housing segment 52 also contains a cable passageway92 which appears on FIGS. 4 and 5 of the drawings.

A fourth housing segment 94 is secured to the lower end of the segment52 and houses the variable speed oscillator motor 30. The pressurepassageway 88 and return passageway 90 are provided through the wall ofthe housing segment 94. Likewise, the cable passageway 92 passestherethrough.

A block member 96 is provided at the lower end of the motor 30 andserves as a manifold for the pressure and return passageways 88 and 90,respectively. A rotary output shaft 98 of the motor 30 is provided withan extension shaft 100 which is journaled in the block 96 and extendsthere through into a fifth housing segment 102. A rotary valve member104 is journaled in the housing segment 102 and is coupled to the rotaryshaft extender 100 by key member 106 as depicted in FIG. 9 of thedrawings. The rotary valve member 104 is cylindrical in shape and isprovided with a pair of spaced longitudinal bores 108 and 110, therethrough.

The lower end of the block member 96 is provided with oppositelydisposed longitudinal bores at the outer periphery thereof which areconnected with and form a part of the pressure and return passageways 88and 90, respectively. The lower end of the bore 88 terminates at thelower end of the blockmember 96 and is in open communication with aninwardly extending groove or recess 112 which terminates with a circularrecess 114 as depicted in FIG. 8 of the drawings. Likewise, the bore 90is in communication with a similar inwardly extending groove or recess116 which terminates in a circular recess 118. The circular recesses 114and 118 are disposed on opposite sides of the center of the lower end ofthe block 96 and are spaced apart by the same distance as thelongitudinal bores 108 and 110 of the rotary valve member 104. The bores108 and 110 are in intermittent communication with recesses 114 and 118and the grooves 112 and 116 when the member 104 is made to rotate.

A longitudinally disposed oscillator chamber 120 is contained within thehousing segment 102 directly below the rotary valve member 104. Anoscillating piston or hammer 122 is slidingly disposed within thechamber 120 for induced oscillation in a manner that will be hereinafterset forth.

Referring to FIGS. 3D and 10 four ports 124, 126, 128, and 130 arespaced 90 degrees apart and are in intermittent communication with thelongitudinal passageways 108 and 110 upon rotation of the rotary valve104. The ports 124 and 126 which are 90 degrees apart are connected inopen communication with the upper end of the oscillatory chamber 120.The port 124 serves as a pressure port while the port 126 serves as areturn port in a manner that will be hereinafter set forth.

The ports 128 and 130 which are 90 degrees apart are connected to thelower end of the oscillatory chamber 120 through longitudinalpassageways 132 and 134 respectively. The passageway 132 serves as areturn passageway while the passageway 134 serves as a pressurepassageway.

In operation, oscillatory motion of the hammer 122 is provided asfollows: Rotation of the rotary valve member 104 is effected by thevariable speed motor 30.

When the rotary valve member is in the position shown in FIG. 9 of thedrawings, air under pressure from the pneumatic reservoir 22 enters thegroove 112 from the passageway 88 in the block member 96. The pressureis then applied through the passageway 108 of the rotary valve 104 intothe port 130 and to the lower end of the oscillatory chamber 120 throughthe passageway 134 causing the hammer or piston 122 to move upwardly orthe the right as shown in FIG. 3D of the drawings.

Exhaust air from the upper end of the oscillatory chamber 120 exits theport 126, through the passageway 110 of the rotary valve 104 and intothe return passageway 90 through the groove 116.

Considering the rotary valve moving in a counterclockwise direction asviewed in FIG. 9 of the drawings, when the passageway 110 comes intoalignment with the port 124, it will also be in alignment with therecess 114 receiving air under pressure from passageway 88 therebyapplying pressure to the upper end of the oscillatory chamber 120causing the piston to move in a downward direction. Exhaust from thelower end of the oscillatory chamber 120 is routed through passageway132 and port 128 wherein it passes back through the rotary valve memberpassageway 108 and into the return passageway 90 of the block member 96.

As the rotary valve continues to move in a counterclockwise direction byan amount of 90 degrees, the process is reversed and repeated therebyimparting oscillatory motion to the hammer member 122. Therefore, it isapparent that for each 360° rotation of the rotary valve member 104, thehammer member 122 completes two oscillations.

Further, it has been found that by proper sizing of the return ports 126and 128, or for that matter anywhere along the return passageways, theexhaust air tends to back up and cause a cushioning or spring effect ateach end of the travel of the oscillating hammer 122. By controlling themass of the oscillating hammer along with the return air passagewayrestriction, the resonant frequency of the oscillating hammer can beadjusted within limits.

It has further been determined that as the speed of rotation of therotary valve member 104 is increased, the efficiency of the routing ofthe air to the oscillating hammer decreases which in ordinarycircumstances would cause the seismic output from the oscillating hammerto decrease as the frequency of oscillation increases. However, byestablishing the resonant frequency of the oscillating hammer systemslightly above the maximum frequency of oscillation, as the hammer nearsits resonant frequency, the output energy is increased thereby causingthe ratio of input energy to output energy to remain substantially flatthereby producing seismic vibrations of somewhat constant amplitude overthe entire frequency range.

It is also noted that the cable passageway 92 extends through thehousing segment 102 as depicted in FIGS. 8 through 11.

The fifth and lower most housing segment 140 contains a clampingapparatus 18 and its associated drive motor 32. The motor 32 is providedwith a downwardly extending drive shaft 144 which is in turn operablyconnected to an elongated particularly threaded drive rod 146 through acoupling device 148.

The lower portion of the housing segment 140 comprises a pair ofoppositely disposed elongated slots one of which is shown in FIG. 16 andis indicated by reference character 150. The lower end of the housing 34is provided with an end plate 152 having a centrally disposed recess 154therein.

The drive rod 146 comprises a lower end threaded portion 156 adjacent toa smooth segment 158. The upper end of the rod 146 has a second smoothshank portion 160 terminating in a polygonal head or wrenching element162.

The coupling member 148 is provided with a lower sleeve member 164 whichhas a polygonal cross sectional shape compatible with the wrenchingelement 162 which serves to impart rotary motion to the drive rod 146while allowing longitudinal movement of said drive rod with respect tothe coupling member 148.

The clamping apparatus 18 further comprises a pair of outwardlyextendable oppositely disposed shoe members 166 and 168 which aretransversely moveable through the elongated slots 150. The outersurfaces of the shoe members 166 and 168 are knurled as indicated byreference character 170 in FIG. 16 to enhance their gripping engagementwith the bore hole wall 12. The lower end of the shoe member 166 isoperably connected to the drive rod 146 by a linkage arm means 172 and asleeve member 174. The sleeve member 174 is threadily carried on thethreaded portion 156 of the drive rod 146 and has a lower portion 176which is reciprocally disposed as a guide member in the recess 154 ofthe end plate 152. The linkage arm 172 is pivotally attached at each endby pins 178 and 180.

Likewise, the lower end of the shoe member 168 is pivotally attached tothe sleeve member 174 by a similar linkage arm 182, the linkage arms 172and 182 being set at an angle with respect to the longitudinal axis toform a scissor-jack arrangement as shown in FIGS. 3E and 14. The smoothcylindrical portion 158 of the drive rod 146 is provided with a sleevemember 184 which is spaced above the sleeve member 174. The drive rod146 is permitted to rotate inside the sleeve member 184 and the sleevemember 184 is held in place on the drive rod by a lower collar member186 with associated pin 187 and an upper elongated sliding plug member188. The plug member 188 is bored to receive the shank portion 160 ofthe drive rod and is provided with a shoulder portion 190 to assure thatthe plug member 188 moves longitudinally with the drive rod 146.

Fluid seals between the clamping member 18 and the drive motor 32 areaccomplished by o-rings 192 and 194 which are carried by the plug member188 and the drive rod shank 160, respectively.

The upper end of the shoe member 166 is pivotally connected to thesleeve member 184 by a linkage arm 196 while the upper end of the shoemember 168 is pivotally connected to the sleeve member 184 by a linkagearm 198. The linkage arms 196 and 198 are again set at an angle withrespect to the drive rod 146 in a scissor-jack arrangement opposite tothe linkage arms 172 and 182.

The arrangement of the linkage arms are such that when the sleevesmembers 174 and 184 are caused to move closer together, the linkage armsforce the clamping shoe members 166 and 168 outwardly into engagementwith the bore hole wall. On the other hand when the sleeve members 174and 184 are moved farther apart, the clamping shoes 166 and 168 areretracted into the elongated slots 150 to assume a position flush withthe configuration of the housing 134 as shown in FIG. 14.

The drive motor 32 is a reversible servo-motor which is capable ofdriving the clamping shoe members to a desired position and holding themembers at that position until the motor 32 is again activated.

In operation, assume that the clamping members start in a retractedposition as shown in FIG. 14 and that the threaded portion 156 of thedrive rod 146 are right-hand threads. Rotation of the drive rod 146 in acounterclockwise direction as shown in FIG. 15 would cause the sleevemember 174 to start moving upwardly or to the right as viewed in FIG. 14thereby forcing the lower end of the clamping shoes 166 and 168outwardly. When the lower portion of the shoes can no longer be extendedor bind up, then rotation of the drive rod 146 causes the drive roditself and the upper sleeve member 184 to move downwardly or to the leftas shown in FIG. 14 thereby forcing the upper portion of the shoemembers 166 and 168 to move outwardly. Rotation of the drive rod 146 iscontinued until the shoes are extended as shown in FIG. 3E.

Rotation of the drive rod 146 in the opposite direction then causes thesleeve members 174 and 184 to move away from each other therebyretracting the shoe members 166 and 168.

Referring now to FIGS. 6A and 6B of the drawings, reference character26A indicates a schematic arrangement of a second embodiment of thedirectional control valve 26 which contains a valve member 200 having aT-shaped passageway 202 intersection, at a solenoid operated valve 203and a second T-shaped passageway 204 intersecting at a second solenoidoperated valve 205.

When the valve member 200 is positioned as shown in FIG. 6A, the returnpassageway 90 is operably connected to the passageway 82 therebyproviding exhaust or return air to the lower side of the pneumaticreservoir 22. Simultaneously the T-shaped passageway 202 providescommunication between the pressure passageway 88 and the passageway 68to the upper end of the reservoir chamber 22.

When the valve member 200 is positioned as shown in FIG. 6B, thelongitudinal passageway 204 operably connects passageways 88 and 80while the T-shaped passageway 202 operably connects passageways 90 and74.

Referring to FIG. 12 reference character 28A generally depicts analternate embodiment of the oscillatory hammer system wherein theoscillatory chamber 120A containing a hammer 122A is operably connectedthrough a two-position valve schematically represented at referencecharacter 206. In the position shown, the pressure in the passageway 88is connected to the upper end of the oscillatory 120A while the lowerend is connected to the exhaust passageway 90.

In a second position with the cross lines being in communication of thepassageways, pressure from the line 88 would be directed to the lowerend of the oscillatory chamber 28 while the upper end would be connectedto exhaust. It is noted that in this arrangement a restriction of theexhaust passageway would be placed in the passageway 90 as opposed tothe oscillatory chamber port.

Referring now to FIG. 13, a third embodiment of the oscillatory hammeris generally represented by reference character 28B, the piston orhammer being designated as 122B while the oscillatory chamber isdesignated as 120B. An alternate oscillatory valve arrangement isindicated schematically by reference character 208. In this arrangementit can be seen that passageways 210 and 212 are designated as pressurepassageways while passageways 214 and 216 serve as exhaust passageways.

From the foregoing it is apparent that the present invention provides asubstantially self-contained down hole periodic seismic generator whichrequires only the electrical power and control lines and support cablingconnected to the well head.

Whereas the present invention has been described in particular relationto the drawings attached hereto, other and further modifications apartfrom those shown or suggested may be made within the spirit and scope ofthe invention.

We claim:
 1. A down hole periodic seismic source for operation within aborehole to produce seismic shear waves through surrounding earth stratacomprisingan elongated housing; means for lowering said housing into aborehole; clamping means carried by the housing for selectively, rigidlyclamping said housing to the wall of the borehole; self-containedpneumatic reservoir means carried by the housing; a longitudinalpneumatic oscillator carried by the housing; and valve means operablyconnected between the reservoir and the oscillator for reciprocallydriving said oscillator longitudinally with respect to the borehole atlow frequencies to produce predominantly vertical shear waves, wherebyvibratory seismic vertical shear waves are transmitted through theclamping means into the surrounding earth strata.
 2. A down holeperiodic seismic source for operation within a bore hole to produceseismic shear waves through surrounding earth strata comprisinganelongated housing adapted for lowering into the bore hole by a cablemeans; clamping means carried by the housing for selectively rigidlyclamping said housing to the wall of the bore hole; pneumatic oscillatormeans carried by the housing for producing vertical seismic vibration inthe housing; a pneumatic pressure source carried by the housing forproviding air under pressure; and a variable speed electro-mechanicaldrive means operably connected between the pneumatic pressure source andthe pneumatic oscillator means for producing seismic vertical shearwaves over a discrete low frequency range.
 3. A down hole periodicseismic source for operation within a bore hole to produce seismic shearwaves through surrounding earth strata comprisingan elongated housing;means for lowering said housing into a bore hole; clamping means carriedby the housing for selectively, rigidly clamping said housing to thewall of the bore hole; self-contained pneumatic reservoir means carriedby the housing; a longitudinal pneumatic oscillator carried by thehousing; and value means operably connected between the reservoir andthe oscillator for reciprocally driving said oscillator longitudinallywith respect to the bore hole at low frequencies to producepredominantly vertical shear waves, whereby vibratory seismic verticalshear waves are transmitted through the clamping means into thesurrounding earth strata and wherein said clamping means comprises aplurality of radially spaced shoe members engageable with the wall ofthe bore hole, a reversible servo motor, and scissor-jack screw meansoperably connected between the servo motor and the shoe members forselectively retracting or extending said shoe members into rigidengagement with the wall of the bore hole.
 4. A periodic seismic sourceas set forth in claim 3 wherein the plurality of shoe members comprisetwo oppositely disposed shoe members.
 5. A periodic seismic source asset forth in claim 4 wherein the scissor-jack screw means comprises acentrally longitudinally disposed threaded rod reciprocally androtatably carried by the interior of the housing, a first end thereofbeing operably connected to the motor, a pair of spaced sleeve memberscarried by the threaded rod and relatively movable with respect to eachother, spaced linkage arms operably connected between each shoe memberand said sleeve members whereby rotation of the rod in one directioncauses relative movement of the sleeve members and linkage arms therebyextending the shoe members and rotation of threaded rods in the oppositedirection causes reverse relative movement of the sleeve members andlinkage arms thereby retracting said sleeve members.
 6. A periodicseismic source as set forth in claim 5 wherein said first end of thethreaded rod is provided with a polygonal wrenching member, andincluding an elongated similar shape polygonal sleeve member carried bythe motor for rotation therewtih, said wrenching member beingreciprocally disposed in the polygonal sleeve member whereby rotation ofthe polygonal sleeve member imparts rotation to the rod while said rodis longitudinally moveable with respect to said sleeve member.
 7. Aperiodic seismic source as set forth in claim 6 wherein one of saidsleeve members is threadily carried by said rod and the second isslidably positioned longitudinally on said rod, the rod being rotatablewith respect thereto, said housing comprising a plurality of transversepassage ways in the housing for directing the transverse travel of eachsaid shoe member.
 8. A down hole periodic seismic source for operationwithin a bore hole to produce seismic shear waves through surroundingearth strata comprisingan elongated housing; means for lowering saidhousing into a bore hole; clamping means carried by the housing forselectively, rigidly clamping said housing to the wall of the bore hole;self-contained pneumatic reservoir means carried by the housing; alongitudinal pneumatic oscillator carried by the housing; and valuemeans operably connected between the reservoir and the oscillator forreciprocally driving said oscillator longitudinally with respect to thebore hole at low frequencies to produce predominantly vertical shearwaves, whereby vibratory seismic vertical shear waves are transmittedthrough the clamping means into the surrounding earth strata and whereinthe selfcontained pneumatic reservoir means comprises an elongatedpneumatic air chamber, an electric motor carried by the housing, arotatable shaft operably connected to the motor and extended into theair chamber and including means for converting rotary motion intoreciprocal motion, an air piston reciprocally disposed in the chamberand operably connected to said shaft and pneumatic porting means carriedby the housing and having a pressure passageway operably connectedbetween the air chamber and the valve means whereby longitudinal travelof said piston forces air under pressure through the pneumatic portingmeans to the valve means.
 9. A periodic seismic source as set forth inclaim 8 wherein the said air piston is double acting and wherein saidpneumatic porting means also comprises an exhaust air return passagewayand a directional control valve means in communication with both ends ofthe air chamber for connecting the return passageway to a low pressureside of the air piston regardless of its direction of travel.
 10. Aperiodic seismic source as set forth in claim 9 wherein the directionalcontrol valve means comprises at least one electrically operatedsolenoid valve.
 11. A periodic seismic source as set forth in claim 9wherein the directional control valve means comprises a plurality ofcheck valves.
 12. A down hole periodic seismic source for operationwithin a bore hole to produce seismic shear waves through surroundingearth strata comprisingan elongated housing; means for lowering saidhousing into a bore hole; clamping means carried by the housing forselectively, rigidly clamping said housing to the wall of the bore hole;self-contained pneumatic reservoir means carried by the housing; alongitudinal pneumatic oscillator carried by the housing; and valvemeans operably connected between the reservoir and the oscillator forreciprocally driving said oscillator longitudinally with respect to thebore hole at low frequencies to produce predominantly vertical shearwaves, whereby vibratory seismic vertical shear waves are transmittedthrough the clamping means into the surrounding earth strata and whereinthe longitudinal pneumatic oscillator comprises an elongated cylindricalchamber, first air passageway means operably connected between a firstend of the chamber and the valve means, second air passageway meansoperably connected between second opposite end of the chamber and thevalve means and an air hammer member reciprocally disposed in thechamber, said valve means being capable of selectively providingpressurized burst of air alternatively through said first and second airpassageway means causing said hammer to oscillate within the chamber toprovide periodic seismic vibration within the housing.
 13. A periodicseismic source as set forth in claim 12, wherein the first airpassageway means comprises a first pressure input passageway and a firstexhaust outlet passageway and the second air passageway means comprisesa second pressure input passageway and a second exhaust outletpassageway.
 14. A periodic seismic source as set forth in claim 12wherein the valve means comprises an electric motor and a rotary valveconnected thereto said rotary valve having two passageways alternativelyconnectable to said first and second air passageway means toalternatively provide pressure and exhaust connections to the airchambers of opposite sides of the hammer.
 15. A periodic seismic sourceas set forth in claim 14 wherein the electrical motor is a variablespeed motor capable of adjustment in speed to provide hammer oscillationover a discrete low frequency range.
 16. A periodic seismic source asset forth in claim 15 wherein said first and second passageway means areported such that some air remains in the chamber during exhaust therebyproviding an air spring with each stroke of the hammer and wherein themass of the hammer and the air spring are designed to produce a resonantfrequency associated therewith, said resonant frequency being just abovethe upper end of said discreet frequency range.
 17. A down hole periodicseismic source for operation within a bore hole to produce seismic shearwaves through surrounding earth strata comprisingan elongated housingadapted for lowering into the bore hole by a capable means; clampingmeans carried by the housing for selectively, rigidly clamping saidhousing to the wall of the bore hole; pneumatic oscillator means carriedby the housing for producing vertical seismic vibration in the housing;a pneumatic pressure source carried by the housing for providing airunder pressure; and a variable speed electromechanical drive meansoperably connected between the pneumatic pressure source and thepneumatic oscillator means for producing seismic vertical shear wavesover a discrete low frequency range and wherein the variable speedelectromechanical drive means comprises a variable speed electric motor,a rotary valve connected to the output of the electric motor andpneumatically connected between the pressure source and the oscillatorwhereby the amount of air provided to the valve decreases as theoscillator frequency increases and including means carried by theoscillator to produce a substantially flat seismic power output oversaid discrete frequency range.
 18. A periodic seismic source as setforth in claim 17 wherein the means carried by the oscillator comprisesa pneumatic oscillator chamber, a reciprocating hammer member disposedin said chamber, porting means carried by the chamber where in thedesign of the mass of said hammer and set porting means combined toproduce a resonant frequency just above the upper end of said discreetfrequency range.